Coherent IQ Modulator Performance

By Winston Way, Ph.D. on March 21, 2016 | Leave a Comment

100/200G coherent pluggable transceiver modules are becoming inevitable for coherent transmission systems, particularly for metro and data center interconnection systems. Just like what happened in the history of 10G pluggable transceivers, the size of coherent pluggable transceivers will become smaller and smaller over time. To enable this size reduction trend, a miniaturized IQ (I: In-phase, Q: quadrature phase) modulator is the key component. Currently, there are three materials for coherent IQ modulators, i.e., lithium-niobate (LiNbO₃), Indium-phosphide (InP), and silicon photonics (SiPho). In this article, we will compare the performance of these three types of IQ modulators based on published data, and focus on only those which have been commercialized or are close to becoming a product.

Today, all three types of commercial coherent IQ modulators are based on parallel inner and outer traveling-wave Mach-Zehnder interferometers (MZI). Each material system uses a different physical property to modulate the light intensity in such a traveling-wave Mach-Zehnder IQ modulator. LiNbO₃ modulators are based on the electro-optic effect, InP modulators are based mainly on the quantum-confined Stark effect, and silicon modulators are based on carrier depletion. Despite the differences in the principles of operation, all three types of modulators have a fundamental limit which can be expressed as Vπ x L = constant, where Vπ is the switching voltage (voltage required to change the modulator output from its maximum to minimum) and L is the active electrode length. A low Vπ, which requires a large L and therefore a higher optical loss, can offer a lower drive voltage and thus a lower electrical power consumption. Conversely, a high Vπ gives a lower optical loss but a higher power consumption. An example is shown in Fig.1 below, where we can see that the higher the optical loss, i.e., a longer interaction between the driving signal voltage and the traveling-wave electrodes, the higher the extinction ratio is. Also, a higher driving voltage can enable a lower optical loss.

Fig.1 Silicon Mach-Zehnder modulator: Optical loss vs extinction ratio and driving voltages. Ref.[1]

Among the three types of modulators, LiNbO₃has been used in telecom optical fiber systems for more than 25 years and has the most mature technology. Its main disadvantage is that it requires a long length of MZI to achieve a reasonable Vπ, which makes it difficult to fit inside a smaller pluggable module such as CFP2-ACO. Consequently, it is currently mainly used on line-cards and in CFP-DCO transceiver modules, which have more available space. InP has been extensively used for MZIs in 10G tunable pluggable transceivers, including small modules such as tunable XFPs and SFPs, so it naturally can fit inside a smaller package such as CFP2-ACO. However, it requires a thermo-electric cooler to stabilize its operation and, therefore, has higher power consumption. Its optical loss is also wavelength-dependent but can be compensated by a DC bias voltage (backplane voltage). SiPho modulators have been implemented in 2-10km 100G CPAK transceiver modules and long-haul coherent CFP-DCO transceivers. However, due to its high optical input/output coupling loss, and the requirement of a low Vπ (therefore higher optical loss in the modulator), it often requires a booster erbium-doped fiber amplifier. In addition, multiple variable optical attenuators (VOAs) or a special DSP algorithm need to be used to compensate for its inherently lower extinction ratio. SiPho modulators, however, can allow wafer-level testing/probing, and can be hermetically sealed at wafer level, which may potentially enable a low-cost optical module package.

Other details to compare the three types of modulators are shown in the table below.

	LiNbO3	InP	SiPho
	Ref.[2]	Ref.[3-7]	Ref.[8]	Ref.[9-10]
Module* or Chip Size	82x10x5.5 mm (module)	2.25×7.45mm (chip)^[3] 37x12x8.1 mm (module)^[2]	27×35.5mm (module, modulator + ICR)	0.4~ 0.625mm long (chip)
Fiber-to-fiber Insertion Loss (per polarization)	<13dB	<13dB	Requires a booster EDFA to compensate for the higher loss	16~17dB
3-dB bandwidth	>23GHz (FPC)	40GHz (chip)^[3] 33GHz (GPPO)^[2]	?	Shows 28Gb/s and 40Gb/s NRZ eye diagrams
Vπ	<3.5V	1.5~2.5V	?	<3.2V
Power Consumption	0	~1W	~0	~0
Extinction Ratio	> 20dB	> 20dB	Requires VOA in each MZI arm or DSP compensation	>9dB
Wavelength-dependent optical loss	0	Yes, requires backplane voltage to keep it <1dB	Yes
DP-QPSK OSNR @ BER=10^-3	14.5dB	14.5~16dB	15.5~17dB	15.2dB
DP-QPSK OSNR @ BER=10^-2		12.5~13dB	13~14.5dB	12.8dB
Requires special DSP to compensate component impairments	No	Yes/No (depends on yield)	Yes	Yes
Manufacturability	Mass production today	Limited production	Limited production	N/A

*Not including boots and fibers

References:

[1] http://www.ieee802.org/3/bm/public/nov12/
gill_01b_1112_optx.pdf

[2] http://www.fujitsu.com/jp/group/foc/en/products/optical-devices/100gln/ & http://www.fujitsu.com/downloads/JP/archive/imgjp/group/foc/services/100gln/ln100gdpqpsk-e-141105.pdf

[3] http://www.teraxion.com/images/
Optical_IQ_modulators_for_coherent_100G_and_beyond_150320-1415e-LightWave.pdf

[4] M. Poirier, et al., “InP Coherent Transmitter for 100Gb/s DP-QPSK Transmission,” OFC 2015, paper Th4F.1.

[5] R.A. Griffin, et al., “InP Coherent Modulator with Integrated Amplification for High Capacity Transmission,” OFC 2015, paper Th4E.2.

[6] R. Blum, “Indium Phosphide enabled coherent pluggable modules for 100G and 200G,” OFC 2-14, Market Watch 100/400G Pluggable Optics and Its Enabling Technologies.

[7] E. Rouvalis, “Indium Phosphide Based IQ-Modulators for Coherent Pluggable Optical Transceivers” pages 1-4, Compound Semiconductor Integrated Circuit Symposium (CSICS), 2015 IEEE.

[8] C. Doerr, et al., “Single chip silicon photonics 100Gb/s coherent transceiver,” OFC 2014, paper Th5C1.1.

[9] B. Milivojevic, et al., “112Gb/s DP-QPSK transmission over 2427km SSMF using small-size silicon photonic IQ modulator and low-power CMOS driver,” OFC 2013, paper OTh1D.1.

[10] M. Webster, et al., “An efficient MOS-capacitor based silicon modulator and CMOS drivers for optical transmitters,” IEEE 11th International Conference on Group IV Photonics (GFP), pp.1-2, 2014.

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